In an aircraft, secondary flight control systems typically include a series of high ratio rotary actuators, each linked to an external flap or slat surface, and all interconnected by a transmission shaft system that delivers power from a centralised Power Drive Unit (PDU). The centralised PDU provides sufficient power to drive all surfaces of the system.
In certain failure cases, all the PDU power can be diverted to one particular actuator, for example in the case of a track jam adjacent to the actuator.
To protect the actuator in question, a torque limiter device is typically incorporated into the actuator input in order to prevent full power from being transmitted into the jammed track. This arrangement is illustrated in FIG. 1.
The torque limiter has to be set to a minimum level that prevents inadvertent torque limiter operation (and consequent system arrest) in normal operation. On the one hand, the airframer wants to minimise this value in order to minimise design loads into the aircraft structure, while on the other hand the system supplier wants to set this value at a high enough level to avoid nuisance tripping of the torque limiter. The margin between these boundaries is further compounded by two factors:                1) Temperature affects the actuator drags and efficiencies. In particular, lubricant within the actuator and the gear boxes that connect the actuator to the PDU drive shaft changes viscosity with temperature. This in turn alters the resistance of these components, i.e. the drag experienced by the drive system.        2) The fundamental efficiency bandwidth of the downstream power stage of the actuator affects the energy absorbed by the drive train between the output shaft of the torque limiter and the actuator. This efficiency changes over time, typically improving as time goes on as the gear teeth wear down and clearances between system components increase, leading to lower resistance.        
The torque limiter's limit must be set sufficiently high that when the efficiency is low and the drag is high (e.g. early in service life and at low temperature) sufficient torque can be transmitted through the torque limiter and the drive train to drive the actuator. However, this high torque limit setting means that when the efficiency is high and the drag is low (e.g. at long life and high temperature), the potential torque transmitted through to the actuator is much higher than required. However, the manufacturer must build the actuator and associated structures with sufficient strength to withstand this potential torque.